Cellular telephony is an outgrowth of the Improved Mobile Telephone Service (IMTS) established in 1964 to provide operator free mobile calls. IMTS was allocated only a smaller number frequency channels and these channels are operated from large master antennas. Accordingly, frequency reuse is limited since frequencies used in one area cannot be used in a neighboring area because of the risk of interference. In addition, since the area covered by an IMTS antenna may be 50 miles in diameter, the long radio link is vulnerable to noise on many calls.
IMTS also implemented multichannel trunking, which allows each user to search and access several two-way channels in order to find one open for calling. Cellular systems advance the multichannel trunking approach by using a computer to automatically search for open channels and establish a call. In cellular technology, frequency reuse in each service area is possible because low-power transmitters and receivers are used to divide the area into cells. Each cell is assigned a set of channel frequencies and arranged so that its neighboring cells use different sets. To avoid co-channel interference between base sites using the same frequencies, antennae heights, transmitter power, and geographic separation are carefully controlled. This arrangement makes possible different conversations over the same frequencies in areas only several miles apart.
As a conversing mobile-phone user travels from one cell to another, the switching center compares signal strength as received at nearby cells. It searches the frequency set of the cell receiving the strongest signal for an open channel and commands the mobile unit to tune to that frequency. With this hand-off of a call to a new cell and channel frequency, the system can accept another call originating in the first cell on the previously occupied channel.
To ensure that a minimum of calls are dropped, cells typically overlap. When a call in progress moves into a busy cell where there are no open channels, it can remain on its original cell until a channel opens or the user moves closer to a third cell with an open channel.
The basic cellular concept has been improved by several different methods which include the following:
Reuse partitioning is a method whereby users with strong signals are assigned to cells with smaller dimensions to permit greater frequency reuse. Dynamic allocation is a method of sharing the frequencies between cells so that cells with high traffic do not get overloaded while there are frequencies in the system that are not being used. The concept of dynamic allocation needs base site transmitters and receivers that can be tuned in frequency or extra transmitters and receivers at sites that anticipate heavy loading. Also, directional antennas can be introduced into cellular systems to reduce the area of coverage and increase the frequency reuse factor. For example, the output power of mobiles can be reduced to thereby reduce interference, or cell patterns can be overlaid on top of other cell patterns to reduce the distance between base sites and effectively reduce the cell size. U.S. Pat. Nos. 4,144,411 and 4,144,496 are examples of how the foregoing concepts are combined in sophisticated systems.
Although the aforementioned prior art systems work well in a theoretical sense, real world problems like buildings, hills and valleys alter propagation and create many combinations that cause cochannel interference. For example, taking a portable into a tall building can potentially cause co-channel interference. For example, bridges that arch up over rivers give mobiles tremendous ranges, and these mobiles cause co-channel interference. Mobiles that drive up hills, and base sites that are located too high on hills cause co-channel interference. In one documented case, a portable phone was used from a private plane, and caused co-channel interference. Monitoring a channel before assignment helps eliminate some of these conditions, but it can also lead to the wrong channel assignments.